1. Field of Invention
The invention relates to a novel method for growing silicon carbide single crystals by liquid phase deposition. More particularly, the invention is concerned with a method for growing silicon carbide single crystals at an increased growth rate by liquid phase deposition using a novel solution.
2. Description of the Related Art
Silicon carbide (SiC) single crystals have excellent physical properties, such as considerably high thermal and chemical stability, high mechanical strength, high radiation hardness, higher breakdown voltage than that of Si, and high thermal conductivity. By adding a suitably selected impurity to the silicon carbide single crystal, it readily provides a p-conductivity-type or n-conductivity-type semiconductor, which has a relatively large forbidden bandwidth (about 3.0 eV in the case of a 6H—SiC single crystal, and about 3.3 eV in the case of a 4H—SiC single crystal). Accordingly, semiconductor devices using the silicon carbide single crystals can be used under high-temperature and high-frequency operating conditions, and are highly resistant to high voltage and harsh environments, though these characteristics cannot be satisfactorily achieved by conventional semiconductor materials, such as silicon (Si) and gallium arsenide (GaAs). Thus, silicon carbide has been increasingly expected as a next-generation semiconductor material.
Typical methods for growing silicon carbide single crystals include, for example, vapor phase deposition or vapor phase epitaxy (VPE), the Acheson method, and liquid phase deposition or solution method. Typical examples of the vapor phase deposition or VPE method include a sublimation process and chemical vapor deposition (CVD). In the sublimation process, various types of defects are likely to be generated in the resultant crystal, and the crystal tends to be polycrystalline. The CVD method uses only gaseous sources as feed materials; therefore, the crystal formed by this method takes the form of a thin film. It is thus difficult to produce a bulk single crystal by the CVD method. The Acheson method uses silica and coke as source materials, which are heated in an electric furnace; therefore, it is difficult or impossible for the resultant crystal to achieve high purity due to the presence of impurities, or the like, in the materials. In an example of method using liquid phase deposition, a silicon-containing alloy is dissolved into a melt in a graphite crucible, and carbon is dissolved from the graphite crucible into the melt, so that a silicon carbide crystal layer derived from the solution is deposited and grown on a seed crystal substrate placed in a low-temperature portion of the solution. Although silicon carbide single crystals are grown at a low rate by liquid phase deposition, in other words, liquid-phase production of silicon carbide single crystals suffers from a low growth rate, it is an advantageous method for producing bulk single crystals. Thus, various studies have been made in recent years (see Japanese Patent Application Publication No. 2000-264790 (JP-A-2000-264790), Japanese Patent Application Publication No. 2004-2173 (JP-A-2004-2173), Japanese Patent Application Publication No. 2006-143555 (JP-A-2006-143555), and Japanese Patent Application Publication No. 2007-76986 (JP-A-2007-76986)), in an attempt to increase the growth rate in the growth of silicon carbide single crystals by liquid phase deposition, which is free from the above-described problems as encountered in vapor phase deposition and the Acheson method.
In a method for producing a silicon carbide single crystal as described in JP-A-2000-264790 as identified above, a source material containing at least one element selected from transition metals, Si and C (carbon) is dissolved into a melt (i.e., C (carbon) is dissolved into the melt which is a solvent containing the at least one element selected from transition metals and Si), with which a silicon carbide seed crystal in the form of a single crystal is brought into contact, and the solution is cooled into a condition where the temperature of the solution is lower than the liquidus line of the solution, so that a silicon carbide single crystal is deposited and grown on the seed crystal. While the transition metals listed by way of example in this publication are Fe, Co, Ni (which belong to the VIII group), Ti, Zr, Hf (which belong to the IVb group), V, Nb, Ta (which belong to the Vb group), and Cr, Mo, W (which belong to the VIb group), only the compositions of the materials containing Mo, Cr, or Co as a transition metal are specifically disclosed. In this publication, there is no disclosure of a method or means for measuring and evaluating the quality and stability of the deposited single crystal.
JP-A-2004-2173 as identified above discloses a melt of an alloy containing Si, and M (M: Mn or Ti) in which, where the atomic ratio of Si and M is represented by Si1-xMx, 0.1≦X≦0.7 when M is Mn, and 0.1≦X≦0.25 when M is Ti. The melt does not contain undissolved C. C is dissolved into the melt from a graphite crucible. In a method for producing silicon carbide single crystal as described in JP-A-2004-2173, a substrate of a silicon carbide seed crystal is dipped into the solution, and the alloy melt around the seed crystal substrate is supercooled so that the solution is supersaturated with silicon carbide, whereby a silicon carbide single crystal is grown on the seed crystal substrate. With regard to the method for producing a silicon carbide single crystal as described in JP-A-2000-264790, it is stated in JP-A-2004-2173 that the silicon carbide produced by this method is likely to be polycrystalline because of the inclusion of carbon in the source material, and the growth rate reaches only 100 μm/h or lower when the temperature of the solution is not higher than 2000° C.
JP-A-2006-143555 as identified above discloses a melt (a solution containing C) of an alloy containing Si, C and M (M: Fe or Co) in which, where [M] is the molar concentration of M and [Si] is the molar concentration of Si, a value of [M]/([M]+[Si]) is equal to or larger than 0.2 and equal to or smaller than 0.7 when M is Fe, and is equal to or larger than 0.05 and equal to or smaller than 0.25 when M is Co. In a method for producing a silicon carbide single crystal as described in JP-A-2006-143555, a seed crystal substrate made of silicon carbide is dipped into the melt of the alloy (the solution containing C), and the alloy melt around the seed crystal substrate is supersaturated with silicon carbide, whereby a silicon carbide single crystal is grown on the seed crystal substrate. In specific examples using the alloys as described above, silicon carbide single crystals are grown at a growth rate of 24.6 μm/h to 75.2 μm/h.
JP-A-2007-76986 as identified above discloses a solution that contains a melt that includes Si, Ti, M (M: Co and/or Mn) as a solvent and C as a solute, and satisfies the relationships of 0.17≦y/x≦0.33 and 0.90≦(y+Z)/x≦1.80 where the atomic ratio of Si, Ti and M is represented by SixTiyMz, and a solution that contains Si, Ti, M (M: Al) and C, and satisfies the relationships of 0.17≦y/x≦0.33 and 0.33≦(y+Z)/x≦0.60 where the atomic ratio of Si, Ti and M is represented by SixTiyMz. In a method for producing a silicon carbide single crystal as described in JP-A-2007-76986, a seed crystal substrate for use in the growth of silicon carbide is brought into contact with the former solution or the latter solution as indicated above, and the solution around the seed crystal substrate is supercooled so as to be supersaturated with silicon carbide dissolved in the solution, whereby a silicon carbide single crystal is grown on the seed crystal substrate. The growth thickness of the silicon carbide single crystal per 100 min. in specific examples using the above-indicated metals ranges from 17.9 μm to 145.0 μm (that corresponds to the range of 10.7 μm/h to 86.8 μm/h when converted into the growth rate).
As is understood from the above description, it is difficult to grow silicon carbide single crystals at a sufficiently high growth rate, by the methods for growing silicon carbide bulk single crystals by liquid phase deposition as described in the above-identified publications.